A 5-year-old Hispanic boy presents to the emergency department having left knee pain of 1 month’s duration. Radiographs of the knee at a clinic were read as normal. There have been no antecedent upper respiratory or constitutional symptoms and no fever. He has experienced intermittent diarrhea and poor weight gain. He has been followed by a hematologist over the previous 8 months for persistent firm splenomegaly and diagnosed by serology as having Ebstein-Barr virus (EBV) infection.

On examination, the child appears well but refuses to walk or bear weight on his left leg. His oral temperature is 100.4°F (38.0°C),pulse is 148 beats/min, blood pressure is 122/69 mm Hg, and respiratory rate is 32 breaths/min. His weight and height are below the 5th percentile. His liver is palpable 2 cm below the costal margin, with a span by percussion of 8 cm, and his spleen is firm and palpable 2 cm...

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